1. Field of the Invention
The present invention relates to a method for curing tyres, comprising the phases of closing a tyre to be cured in the moulding and curing cavity of an appropriate mould and of heating the lower, upper and lateral walls of said mould to transmit heat to the tyre.
The invention further relates to a device for curing tyres, comprising: a curing mould defining a moulding and curing cavity suitable to receive a tire being processed at least a lower hot table situated adjacently to a lower wall of said curing mould; at least an upper hot table situated adjacently to an upper wall of said curing mould, at least a peripheral hot table situated around the lateral development of the moulding and curing cavity; devices for supplying heating fluid in said hot tables.
2. Description of the Related Art
Production processes for vehicle tyres provide for each tyre, after having been assembled in all its components, to be subjected to a moulding and curing process during which the tyre itself is made to adhere with adequate pressure against the inner walls of a moulding cavity, and to the administration of heat to determine, with the polymerisation of the elastomeric material whereof it is composed, its definitive geometric and structural stabilisation.
With particular reference to the curing of tyres for vehicles, for this purpose the use of so-called centripetal devices is widespread. Examples of such devices are for instance described in the following patents: U.S. Pat. No. 5,676,980, EP 123,733, EP 170,109 and EP 459,375.
Devices of this kind are generally arranged with the axis of the mould in vertical position and the median, or equatorial plane, of the mould horizontal relative to the ground; the moulding cavity is essentially defined between two annular cheeks, respectively inferior and superior with reference to said plane, coaxially arranged to form the exterior surfaces of the sidewalls of the tyre. To the cheeks is combined a crown of sectors arranged circumferentially around the geometric axis of the mould, coinciding with the axis of rotation of the tyre, to shape the tread band of the tyre, define therein a series of indentations and/or grooves arranged in varied ways according to a desired tread design.
The lower cheek is generally fixed relative to a base, whereas the upper cheek, engaged to a so-called lid movable vertically, can be translated axially relative to the lower cheek.
The circumferential sectors, in turn, are slidingly engaged to a sector-holder ring which is integral, depending on the case, either with the lid or with the base. More in particular, the sectors are movable along respective generatrices of a cone frustum shaped surface provided in the sector-holder ring. Simultaneously with the mutual axial actuation between ring and sectors, the latter are simultaneously translated radially to the axis of the mould, between an open condition wherein they are circumferentially distanced from each other, i.e. radially removed from the outer circumferential surface of the tyre contained in the mould, and a closed condition wherein they are radially approached and mutually matching in correspondence with the respective circumferentially opposite edges.
Hearing devices are further provided for transmitting a pre-determined quantity of heat to the tyre closed in the moulding cavity, in order to determine its adequate curing.
These heating devices essentially comprise a plurality of so-called “hot tables” positioned around the components that define the moulding and curing cavity. For the purposes of the present description, the term “hot table” is taken to mean a structural part of the curing unit, presenting at least a cavity, different from the moulding and curing cavity, able to be supplied with any suitable heating fluid.
More specifically, a lower hot table, located immediately underneath the aforesaid lower cheek, an upper hot table, located adjacently above the upper cheek, and an annular hot table associated with the sector-holder ring and extending around the sectors themselves, are provided.
Whilst the upper and lower hot tables are supplied with fluid, usually steam, at the same temperature, in the order of 175° C. by way of indication, the fluid injected into the annular hot table can have a temperature in the order of 185° C., or in any case exceeding the temperature of the fluid injected into the lower and upper chambers.
This greater temperature is selected in consideration of the fact that the crown of the tyre, in contact with the sectors, has a greater thickness than the thickness of the sidewalls and includes the belt structure, i.e. a pack of at least three strips of rubber-coated fabric, textile or metallic, and thus requires a greater quantity of heat to reach, in the same cycle time, substantially the same degree of curing.
In particular, it is also known document GB 1,118,005 disclosing an apparatus for embossing a pattern in the tread band of a pneumatic tyre of the type described above which may supply steam at different temperatures to the chambers of the hot tables in dependence of the thickness of the parts of the tire to be cured.
It has always been deemed that the known curing method described above allowed to fabricate tyres with a homogeneous degree of curing or, at least, with any inconsistencies symmetrically distributed relative to the equatorial plane in relation to the different thickness between the different areas of the tyre and to the different size of the carcass in said areas.
After the premise above, it should be noted that the tyre industry constantly strives to search for continuous improvements both of the performance offered by the tyre in operation and of the degree of uniformity of the product in order continually to restrict the band of tolerability of the values of the viscosity and elasticity characteristics of the cured tyre which deviate from the pre-determined ideal value.
In this search for continuous improvements, sustained by an accurate analysis of the product, the Applicant has found that the tyre often behaves differently, to a greater or lesser extent, when engaged in a curve in a given direction from the way it does when engaged in the opposite direction.
This inconsistent behaviour was initially and hastily attributed to tolerable quantitative differences between the geometric characteristics (thickness, height and profile) of the two semi-finished products that constituted the opposite sidewalls of the tyre or to small variations in the geometric characteristics of the compound of the two sidewalls which took place during the sidewall preparation (straining or calendering) process.